Receivers for optical signals are useful in a wide variety of applications including optical communication systems. As presently contemplated, such systems typically have a light source and receiver optically coupled to each other by means of an optical fiber pathway. Information is usually transmitted by varying the intensity of optical radiation and the rate of variation is referred to as the bit rate. An optical receiver detects an incoming optical signal and either regenerates the optical signal or converts it to an electrical signal.
The optical receiver should have a high sensitivity. The optical receiver should also have a large dynamic range. Additionally, typical optical communications systems operate within a wide range of bit rates. A common approach for achieving high sensitivity and wide bandwidth for an optical receiver, is to use a transimpedance amplifier. (See, e.g., Ogawa, "Considerations For Optical Receiver Design" IEEE Journal on Selected Areas in Communications" , VOL SAC-1, No. 3, April 1983, pp. 524-532). This type of amplifier has a resistance to provide electrical feedback between the amplifier output and input. With this configuration, the feedback resistance must be very large so that the Johnson noise from the feedback resistance is less than the amplifier noise. Such a large resistance causes several problems. For example, the dynamic range of the receiver is limited because very large amplifier output voltage swings are needed for an appreciable current flow through the feedback resistor. Additionally, the bandwidth is limited because any parasitic capacitance in parallel with the feedback resistance produces a large RC time constant.
A receiver utilizing optical feedback between the amplifier output and the amplifier input eliminates many of the problems associated with the feedback resistance in a transimpedance amplifier while providing the necessary current feedback to achieve wide dynamic range. Such a receiver comprises an operational amplifier having a positive and a negative input and an output. Illustratively, the negative input of the operational amplifier is connected to a voltage -V/2. Connected to the output of the operational amplifier via a resistor is a light source such as an LED. Connected to the positive input of the operational amplifier is a feedback photodetector, illustratively in the form of a P-I-N photodiode. There is an optical path between the light source and feedback photodetector. There is also a signal photodetector connected to the positive input of the operational amplifier. The signal photodetector and the feedback photodetector are connected in series with the anode of the signal photodetector being connected to a voltage -V and the cathode of the feedback photodetector being connected to ground. The signal photodetector receives the incoming optical signal. During receiver operation, negative feedback as a result of optical transmission between the light source at the amplifier output and the feedback photodetector insures that the optical level at the feedback detector is large enough so that the photo-current in the feedback detector balances the photo-current in the signal detector.
The use of optical feedback has a number of significant advantages in comparison to the conventional transimpedance amplifier. In particular, the optical feedback coupling eliminates the feedback resistance as a noise source and the parasitic capacitance as a bandwidth limitation. It also insures that the amplifier output voltage swing is small resulting in a wide dynamic range.
Thus, the use of optical feedback in an optical receiver provides significant advantages over the use of a resistance to define a feedback path.
Presently, receivers using optical feedback are implemented with discrete light emitting and photodetecting components in the feedback path (see, e.g., U.S. Pat. No. 4,744,105; U.S. Pat. No. 3,955,149; U.S. Pat. No. 4,284,960; U.S. Pat. No. 4,625,105; U.S. Pat. 3,611,173; UK patent application no. 2,030,020A and German Patent Document No. 2, 218,431).
In contrast to the use of discrete components to implement a light source and photodetector to provide a feedback path for a receiver, it is an object of the present invention to provide a single integrated semiconductor device including a light emitting diode and a P-I-N photodetector for use in providing an optical feedback path for a receiver.
Toward this end, it is useful to consider a multiple quantum well structure. While discrete multiple quantum well lasers and modulators are well known (see, e.g., Daniel S. Chemia "Quantum Wells for Photonics" Physics Today, May 1985, pp 52-64; U. Koren et al "Low Internal Loss Separate Confinement Heterostructure InGaAs/InGaAsP Quantum Well Laser" Appl. Phys. Lett 51(21) 23 Nov. 1987 1744-1746; E. Zielinski et al "Optical Gain and Loss Processes in GaInAs/InP MQW Laser Structures" IEEE Journal of Quantum Electronics Vol. 25, No. 6 June 1989 pp 1407-1406, U. Koren et al "Low-loss InGaAs/InP Multiple Quantum Well Optical Electroabsorbtion Waveguide Modulator" Appl. Phys. Lett. 51(15) 12 Oct. 1987, pp 1132-1134; T. H. Wood "Multiple Quantum Well (MQW) Waveguide Modulator" IEEE Journal of Light Wave Technology Vol.6, No. 6 June 1988 pp 743-757), a single integrated multiple quantum well structure including both a light emitter and a photodetector for providing an optical feedback path for a receiver has heretofore not been realized. Accordingly, it is a further object of the invention to provide a single integrated multiple quantum well structure which includes both a light emitter and a photodetector for implementing an optical feedback path for a receiver.